A comparative analysis of standard accretion disks spectra: an application to Ultraluminous X-ray Sources

TL;DR

This paper compares standard accretion disk models with more physical ones, providing correction recipes to better estimate black hole parameters from X-ray spectra, and applies these to ultraluminous X-ray sources.

Contribution

It introduces simple correction recipes for accretion disk parameters, improving estimates of black hole mass and accretion rate from spectral fits.

Findings

Black hole masses for some ULXs are around 100-200 solar masses.

Maximal black hole rotation implies larger mass estimates.

Standard disk models can be adjusted for more accurate physical parameters.

Abstract

We compare standard models of accretion disks around black holes that include the appropriate zero-torque inner boundary condition and relativistic effects on the emission and propagation of radiation. The comparison is performed adopting the multicolor disk blackbody model (MCD) as reference and looking for the parameter space in which it is in statistical agreement with "more physical" accretion disk models. We find simple 'recipes' that can be used for adjusting the estimates of the physical inner radius of the disk, the black hole mass and the accretion rate inferred using the parameters of the MCD fits. We applied these results to four ULXs for which MCD spectral fits of their X-ray soft spectral components have been published and find that, in three cases (NGC 1313 X-1, X-2 and M 81 X-9), the black hole masses inferred for a standard disk around a Schwarzschild black hole are in the interval ~100-200 solar masses. Only if the black hole is maximally rotating are the masses comparable to the much larger values previously derived in the literature.